In Situ Transmission Electron Microscopy Imaging of Nanomaterials Growth and Transformations

An understanding of nanoscale materials transformations is significant for the synthesis and applications of nanostructured materials. At the atomic level, heterogeneity and fluctuations may play a governing role in the growth and transformation processes. The recent development of high-resolution liquid cell transmission electron microscopy (TEM) has enabled breakthroughs in our ability to follow the atomic level structural, morphological, or chemical changes of materials. Thus, it has provided an unprecedented opportunity to resolve the growth and transformation mechanisms of materials that cannot be predicted based on thermodynamic equilibrium. I will show our recent development of high resolution liquid cell TEM for imaging of nanoscale materials growth and transformations. With the high resolution liquid cell TEM, we have been able to study the defect-mediated ripening of Cd-CdCl₂ core-shell nanoparticles (CSN). Ripening was found to start by dissolution of the nanoparticle with an incomplete CdCl₂ shell, and that the Cd core that was exposed to the solution dissolved first. The growth of an Cd-CdCl₂ CSN was achieved by generating crack defects in the shell, followed by ion diffusion through the cracks. The healing of crack defects led to a highly crystalline CSN at the end. The formation and annihilation of crack defects in the shell, accompanied by disordering and crystallization of the shell structure, mediated the ripening of CSN in the solution. I will also show our in-situ high resolution liquid cell TEM study of the nucleation of CdS nanoclusters. The growth of semiconductor nanocrystals is classical in colloidal nanocrystal synthesis, and it has been studied for many years. However, this is the first time that we were able to achieve direct observe the growth of semiconductor nanocrystals. Many fascinating dynamic phenomena were achieved, and they provide critical insights on their nucleation and growth mechanisms.